Efficient Electric Spacecraft Propulsion

ABSTRACT

A propulsion system for spacecraft is based on an electric engine that expels propellant to achieve thrust. The propellant is first ionized to generate a plasma. Plasma particles are selectively accelerated via a pulsed laser that accelerates predominantly the electrons in the plasma. The electrons are expelled first, forming a space charge that acts as a virtual cathode to accelerate the positive ions. Interactions between the laser beam and plasma electrons are predominantly through the ponderomotive force.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/209,249, filed on Mar. 13, 2014, entitled “Hybrid Electric Propulsionfor Spacecraft”, and claiming the benefit of U.S. provisional patentapplication No. 61/779,378. The complete disclosure of both theabove-identified patent application and the provisional patentapplication are expressly incorporated herein, by reference, in theirentireties.

FIELD OF THE INVENTION

The present invention relates to electric spacecraft propulsion ingeneral, and, more particularly, to ion engines and plasma engines.

BACKGROUND OF THE INVENTION

Electric propulsion for spacecraft is highly desirable for a variety ofreasons, and, through the years, many designs for electric engines forspacecraft have been proposed and implemented. Two important categoriesof electric spacecraft engines are the ion engines and the plasmaengines.

Before the advent of electric spacecraft engines, chemical rockets werethe only technology available for spacecraft propulsion, and, the earlydecades of space exploration were based almost entirely on chemicalrockets. An important difference between chemical rockets and electricengines is that, with chemical rockets, fuel and propellant are one andthe same, but not so with electric propulsion. In this disclosure, theterm “fuel” is used exclusively to refer to the source of energy for apropulsion system, while the term “propellant” is used exclusively torefer to the mass that is expelled by a propulsion system.

Most spacecraft propulsion systems accomplish their task by exploitingthe principle of conservation of momentum. In accordance with thatprinciple, if a mass is expelled by a spacecraft, there will be a forceacting on the spacecraft while the mass is being expelled. Such force isreferred to as “thrust” and is in proportion to the amount of mass thatis expelled and in proportion to the velocity at which the mass isexpelled. The higher the rate at which mass is expelled, the higher thethrust. Similarly, the faster the velocity of expulsion of the mass, thehigher the thrust.

From the principle of conservation of momentum, as described in theprevious paragraph, it might seem that a high velocity of expulsion isdesirable because it yields higher thrust with less mass being expelled.However, the higher velocity comes at the cost of higher energy. Inparticular, calculations show that the power that must be expended toachieve a certain amount of thrust increases in proportion to thevelocity of expulsion.

In spacecraft design, reducing the amount of propellant that needs to becarried is highly desirable. Such a reduction can be achieved byincreasing the velocity of expulsion of the propellant. But reducing theamount of energy required by the spacecraft is also highly desirable,such that, in each space mission, a compromise must be struck betweenpropellant mass and required energy. Such a compromise depends on thespecific parameters of the mission and may be different in differentparts of the mission. Therefore, it is advantageous to have a propulsionsystem wherein the velocity of expulsion of the propellant can beadjusted as needed to achieve such a compromise.

With chemical rockets, the velocity of expulsion of the propellant islimited by the amount of energy available from chemical reactions.Generally, the velocity of expulsion is much less than the optimum inmost circumstances. That's why there is no benefit, with chemicalrockets, in carrying propellant in addition to the chemical fuel neededfor providing the chemical energy. Calculations show that best resultsare obtained by using the spent chemical fuel as propellant withoutmixing in additional propellant. That's also why the mass of a spacerocket sitting on the launch pad is so much larger than what eventuallymakes it into orbit. Most of that mass is fuel.

Electric spacecraft engines are advantageous because fuel and propellantare separate. In particular, energy is supplied to the engine aselectricity, and the fuel from which the electricity is generated cancome from, for example, a nuclear reactor or a radioisotope source. Bothsuch forms of fuel yield much more energy, per unit mass, than chemicalfuels, such that there is no need to use the spent fuel as propellant.Better yet, for spacecraft that are close enough to the Sun, there isthe option of generating electricity with solar panels. In such a case,the fuel is located in the Sun, and the spacecraft does not need tocarry any fuel.

One potential benefit of electric engines is the opportunity to adjustthe velocity of expulsion of the propellant. The feasibility of such anadjustment depends on the design of the electric engine, and somedesigns are better than others in that respect.

There are two major categories of electric engines known in the art:plasma engines and ion engines. With both categories, the propellant isprepared for expulsion by first ionizing it. The ionized propellantforms a plasma, which is a state of matter wherein atoms have lost oneor more electrons, thereby becoming positive ions; and wherein the lostelectrons remain mixed in with the ions, such that the overall mixturehas no net electric charge.

Plasma engines heat the plasma by any of a variety of techniques wellknown in the art. As with any substance, heating a plasma means thatplasma particles are accelerated, such that their kinetic energy (KE)increases. Heating implies that the resulting motion of plasma particlesis random, so that different particles have different kinetic energiesin accordance with a random distribution, and the direction of motion ofthe particles is also random with no preference for any particulardirection of motion. Some of the heated plasma is then allowed toescape, forming the expelled propellant. Generally, the temperature ofthe plasma determines the average velocity with which the particlesescape, which is the velocity of expulsion.

In contrast, with ion engines, ions and electrons in the plasma areseparated via electric fields without heating the plasma. Electricfields are further used to accelerate the separated ions and electronsfor expulsion. Ions and electrons are then expelled separately, and theyrecombine outside the spacecraft, forming the expelled propellant.

Plasma engines and ions engines have different advantages anddisadvantages. With ion engines, the use of electric fields makes itpossible to adjust the velocity of expulsion accurately by adjusting theelectric fields that accelerate the ions and electrons. This capabilitymaximizes the efficiency of utilization of propellant. Also, the size ofthe flow of electrons and ions can be similarly adjusted, such that thethrust generated by the engines can be adjusted easily and accurately.Another advantage is simplicity, because the conversion of electricenergy into electric fields can be simply accomplished with metalelectrodes that have specific shapes. However, there are disadvantageswith ion engines. In particular, the accelerated ions might come indirect contact with the material of the electrodes and cause damage tothe electrodes. And the negative electrons that leave the cathode andimpinge on the anode also can cause damage to those electrodes.Generally, electrodes in ion engines have limited lifetime due to suchwear.

Plasma engines are advantageous because they are not subject to the samewear mechanism as ion engines. But they are more complex because of theneed to convert electric energy into plasma heating, and because therandom motion of heated plasma particles, as they escape, means that notall particles have the optimal velocity of expulsion.

It would be desirable to have a type of electric spacecraft engine thatcombines the advantageous features of plasma engines and ion engines.

SUMMARY OF THE INVENTION

Embodiments of the present invention provide advantages of plasmaengines in combination with advantages of ion engines. They do so bycombining features of both engines and by employing an altogetherdifferent technique for accelerating plasma particles. In particular,electric spacecraft engines in accordance with the present inventionaccelerate plasma particles by means of one or more laser beams.

The parameters of the interaction between a laser beam and plasmaparticles are adjusted such that energy is transferred from laser lightto plasma particles predominantly through the ponderomotive force. Sucha force acts almost exclusively on the electrons in the plasma, and itresults in the electrons being accelerated in a direction predominantlyperpendicular to the direction of propagation of the beam. In contrast,the ions in the plasma are much less sensitive to the ponderomotiveforce than the electrons, and they undergo only a negligible amount ofacceleration.

In embodiments of the present invention, the vessel containing theplasma has one or more outlets for letting some of the plasma particlesescape. By appropriately positioning the laser beam relative to theoutlets, the electrons accelerated by the ponderomotive force movepreferentially toward one or more of the outlets. This is in contrast toprior-art plasma engines which just heat the plasma. Compared to plasmaheating, the ponderomotive force, as generated in embodiments of thepresent invention, does not accelerate all plasma particles equally, andnot in random directions; instead, it accelerates electrons almostexclusively, and the electrons are accelerated in a directionpredominantly perpendicular to the direction of propagation of the laserbeam.

Embodiments of the present invention also comprise electrodes forgenerating electric fields. Such electrodes are positioned near theoutlets, and are arranged such that they generate electric fields thataffect the flow of plasma particles. In particular, when a voltage isapplied to the electrodes, the resulting electric field constitutes abarrier to the flow of plasma particles, and the height of the barriercan be adjusted by adjusting the voltage. As the electrons areaccelerated by the ponderomotive force, they acquire increasinglygreater kinetic energy until they become able to overcome the barrierand flow through one or more of the outlets. In contrast, the ions,which are largely unaffected by the ponderomotive force, remain insidethe vessel containing the plasma.

In prior-art ion engines, the positive ions in the plasma areaccelerated by a negatively-charged cathode. In embodiments of thepresent invention, the positive ions are accelerated by a virtualcathode. In particular, electrons, of course, are negatively charged.Therefore, as the electrons flow through the outlets and startaccumulating outside of the vessel, they form a negative space chargethat occupies a volume of space outside the outlets. This negative spacecharge behaves like the negatively-charged cathode in prior-art ionengines, and attracts the positive ions in the plasma. This attractioncauses the ions to accelerate in the direction of the space charge, suchthat they, too, escape the plasma vessel through one or more outlets andare accelerated in the direction of the space charge.

Upon reaching the electron space charge, the positive ions recombinewith the electrons, thus forming the expelled propellant.

In embodiments of the present invention, the arrangement of electrodesnear the outlets is such that the resulting electric fields favor theflow of negatively-charged electrons through some outlets, or favor theflow of positively-charged ions through other outlets. The outlets canbe positioned, relative to the laser beam, such that the flow ofelectrons and ions through the respective outlets is further enhanced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram that illustrates the structure of a hybrid electricspacecraft engine in accordance with an illustrative embodiment of thepresent invention.

FIG. 2 depicts a perspective view of a plasma containment vessel.

FIG. 3 is a diagram that illustrates what happens when a plasmagenerator is activated in a hybrid electric spacecraft engine inaccordance with an illustrative embodiment of the present invention.

FIG. 4 depicts a hybrid electric spacecraft engine with a laser beamilluminating a plasma cloud.

FIG. 5 depicts a hybrid electric spacecraft engine with electronsescaping out of a plasma containment vessel through an outlet.

FIG. 6 depicts a hybrid electric spacecraft engine with electrons andions escaping out of a plasma containment vessel through two outlets.

FIG. 7 a flow diagram of a method for generating thrust via a hybridelectric spacecraft engine.

FIG. 8 shows two possible paths that plasma particles might follow whenescaping from a plasma containment vessel.

FIG. 9a is a graph that shows the electrical potential along a path forparticles escaping from a plasma containment vessel.

FIG. 9b is a graph that shows the electrical potential along a path forparticles escaping from a plasma containment vessel.

DETAILED DESCRIPTION

FIG. 1 is a diagram that illustrates the structure of hybrid electricspacecraft engine 100 in accordance with an illustrative embodiment ofthe present invention. Hybrid electric spacecraft engine 100 comprisesplasma containment vessel 110, plasma generator 120, laser 130 andelectrodes 140-1 through 140-8, interrelated as shown.

Plasma generator 120 is a device for generating a plasma. It iswell-known in the art how to make such a device. For example, such adevice might comprise a spark generator for generating an electric sparkacross the face of a solid surface. The heat of the spark causes some ofthe solid material to vaporize, and then the mobile electrons of thespark ionize the particles of the vaporized material. Such ionizationresults in the vaporized material becoming a plasma comprised of freeelectrons and free positive ions. For example, the solid material mightbe made of carbon, in which case the positive ions are carbon atoms thathave lost one electron. The mass of one such positive ion is, as is wellknown in the art, approximately 22,000 times as much as the mass of oneelectron; but the positive ion has an electric charge that is the sameas the charge of one electron, with the opposite sign.

Plasma containment vessel 110 is a vessel for confining the plasmagenerated by plasma generator 120 to a finite volume of space. It iswell known in the art how to make such a containment vessel. Forexample, plasma containment vessel might be made of acrylic material. Inthe illustrative embodiment of the present invention represented in FIG.1, plasma containment vessel 110 is made of such material. It will beclear to those skilled in the art, after reading this disclosure, how tomake and use plasma containment vessels made of other materials. Forexample, a class of plasma containment vessels well-known in the art ismade not out of ordinary matter, but rather out of magnetic fields. Suchvessels take advantage of the fact that a plasma is a collection ofcharged particles whose movements are affected by electric and magneticfields. Of course, such a magnetic vessel requires magnets or magneticcoils made out of ordinary matter for generating the magnetic fields.For example, superconducting coils might be used for such purpose.

Outlets 150-1 and 150-2 are openings in one of the walls of plasmacontainment vessel 110. They are for allowing some of the plasmaparticles to escape and, thereby, provide propulsion, as explained ingreater detail in the coming paragraphs.

In this illustrative embodiment of the present invention, electrodes140-1 through 140-8 are wires that are embedded in the wall of plasmacontainment vessel 110; they are placed in the vicinity of openings150-1 and 150-2 for the purpose of generating electric fields in andaround the openings. They are shown in cross section in FIG. 1.

FIG. 2 depicts a perspective view of plasma containment vessel 110, asviewed from the outside. The two outlets 150-1 and 150-2 are visible asopenings in the front wall of the vessel, as depicted in the figure. Inthis depiction, the wires of electrodes 140-1 through 140-8 are notvisible because they are embedded in the acrylic material of which thevessel is made, but they run vertically, as depicted in the figure,along the two openings.

It is an important advantage of the present invention that theelectrodes are entirely embedded in the acrylic material, such that theynever come in contact with plasma particles. Therefore, in contrast withion engines, there is no electric current flowing through electrodes140-1 through 140-8, and they do not experience any damage due tointeractions with plasma particles.

FIG. 3 is a diagram that illustrates what happens when plasma generator120 is activated. It is a feature of the present invention that plasmagenerator 120 is not continuously active. Instead, the plasma generatoris activated for a short period of time in order to generate a cloud ofplasma, depicted in FIG. 3 as plasma cloud 160, inside plasmacontainment vessel 110. In embodiments of the present invention, plasmagenerator 120 provides enough energy to ionize the plasma, but not muchmore than that.

FIG. 3 also shows that, prior to activating the plasma generator, avoltage is applied to electrodes 140-1 through 140-8. In particular,electrodes 140-1, 140-2, 140-7, and 140-8 are connected to the positiveterminal of a constant-voltage generator, and electrodes 140-3 through140-6 are connected to the negative terminal of the constant-voltagegenerator. Such connections are indicated in the figure by “+” or “−”symbols inside the circles that represent the electrodes.

The constant-voltage generator is not shown explicitly in FIG. 3. It iswell known in the art how to make a constant-voltage generator. Forexample, such a generator might be a direct-current (DC) power supplythat generates a suitable voltage. As already mentioned, it is a featureof the present invention that, nominally, no current flows throughelectrodes 140-1 through 140-8, such that, nominally, the voltagegenerator does not have to provide any power, just a fixed voltage. Inpractice, the acrylic material is not a perfect insulator, and somesmall leakage current is likely to be present. Therefore, the DC powersupply needs to be kept turned on during normal operation in order tokeep the electrodes continually polarized; however, the amount of energythat the DC power supply must deliver is very small, just enough forcounteracting the leakage current.

The voltage applied to electrodes 140-1 through 140-8 results in thecreation of electric fields in the volume of space inside outlets 150-1and 150-2. Such electric fields are a barrier to the flow of plasmaparticles, such that the plasma, as generated by the plasma generator,remains inside the containment vessel. This is so because, as notedabove, the plasma generator provides only enough energy to ionize theplasma, but not much beyond that. As a result, the kinetic energy ofplasma particles is not sufficient for them to overcome the barrierspresented by the electric fields in the two outlets.

After plasma generator 120 has generated plasma cloud 160 inside plasmacontainment vessel 110, laser light is used to accelerate plasmaparticles. In particular, laser 130 is activated to generate a laserbeam.

FIG. 4 depicts hybrid electric spacecraft engine 100 with the laser beamilluminating the plasma cloud. The laser beam is depicted as laser beam170. The laser beam illuminates a portion of the plasma cloud, such thatthe plasma particles that are in the path of the beam experience aponderomotive force.

In physics, the ponderomotive force is defined as the force that acharged particle experiences in an inhomogeneous oscillating electricfield such as the electric field present in a laser beam. In particular,as the laser light propagates in the laser beam, there is a strongoscillating electric field in the center of the beam. A plasma particlelocated at or near the center of the beam experiences this oscillatingfield; however, the amplitude of the field vanishes if the particle ismoved outside of the beam. This decrease in amplitude is aninhomogeneity of the oscillating electric field, and, therefore, plasmaparticles that are inside the laser beam experience a ponderomotiveforce that accelerates them in a direction perpendicular to thedirection of propagation of the beam.

The formula for the ponderomotive force is

$F = {{- \frac{e^{2}}{4m\; \omega^{2}}}{\nabla E^{2}}}$

wherein F is the strength of the ponderomotive force; e and m are,respectively, the charge and the mass of the charged particle; w is theangular frequency of the oscillating field, and E is the peak amplitudeof the oscillating electric field. It is important to note that the massof the charged particle appears in the denominator. Therefore, in aplasma wherein electrons are much lighter than positive ions, theeffects of the ponderomotive force are felt much more strongly by theelectrons than by the positive ions. For example, if the positive ionsare carbon ions, the energy acquired by the electrons because of theponderomotive force is about 22,000 times as much as the energy acquiredby the ions.

In this illustrative embodiment of the present invention, the laser is apulsed laser; i.e., it is a laser that emits light as a sequence ofshort periodic pulses. Each pulse acts on the electrons in the plasmathrough the ponderomotive force, with negligible effects on the positiveions because of their larger mass. Each pulse accelerates the electronsin a direction perpendicular to the direction of propagation of the beamand this acceleration results in an increased kinetic energy of theelectrons.

As the kinetic energy of the electrons increases after each laser pulse,at some point they will have acquired enough kinetic energy to overcomethe barrier presented by the electric fields in outlets 150-1 and 150-2.In particular the polarity of the electric fields in outlet 150-2 issuch that the electrons need less kinetic energy to overcome it, and,therefore, they will start flowing through that outlet as soon as theirkinetic energy is large enough.

FIG. 5 depicts hybrid electric spacecraft engine 100 with electronsescaping out of the plasma containment vessel through outlet 150-2. Theescaping electrons are depicted as electron flow 180-1.

Outlet 150-2 opens into outer space, such that the electrons that escapethrough the outlet leave the spacecraft entirely. However, electrons arenegatively charged, while the positive ions that are left behind are, ofcourse, positively charged. As more and more electrons leave thespacecraft, the spacecraft acquires a positive charge because of thepositive ions left behind. Therefore, the negative electrons areattracted back toward the spacecraft, and never get too far from it.

As more and more electrons accumulate outside the spacecraft, in thevolume of space just outside the two outlets 150-1 and 150-2, they forma negatively-charged cloud referred to as a “space charge”.

The presence of the negative space charge on the outside of thecontainment vessel and in front of the outlets, together with the cloudof positively charged ions that were left behind in the containmentvessel, on the other side of the outlets, alters the shape of theelectric field in outlet 150-1; and the attraction of the positive ionsby the negative space charge pushes the ions in the direction of theoutlet with enough kinetic energy to overcome the barrier in thatoutlet.

Much like the polarity of the electric fields in outlet 150-2 favoredthe flow of electrons, the polarity of the electric fields in outlet150-1 favors the flow of positive ions, such that, when the space chargehas accumulated enough electrons, positive ions start flowing throughoutlet 150-1 while being accelerated by the electric fields in theoutlet.

Like outlet 150-2, outlet 150-1 also opens into outer space, in the samedirection as outlet 150-2. Therefore, the positive ions that areaccelerated by the electric fields in outlet 150-1 escape into the samevolume of outer space where the space charge is present. There, theyrecombine with the electrons, and, due to the kinetic energy that theyhave acquired, they continue moving away from the spacecraft withoutcarrying any electric charge.

The electron space charge in this illustrative embodiment of the presentinvention behaves similarly to the cathode of an ion engine. Therefore,it is referred to as a “virtual cathode” and is depicted in FIG. 5 asvirtual cathode 190. However, in contrast to an ion engine where thecathode is made out of an electrically conductive material, the spacecharge is not damaged by the flow of electrons and positive ions.

FIG. 6 depicts hybrid electric spacecraft engine 100 with electrons andions escaping out of the plasma containment vessel through outlets 150-1and 150-2. The escaping ions are depicted as ion flow 180-2. The figureshows that the escaping ions meet the electrons in the space charge,where they recombine and, by virtue of their kinetic energy, continuemoving away from the spacecraft, thereby generating thrust through theprinciple of conservation of momentum. After allowing enough time for asubstantial number of positive ions to escape, the process is repeatedstarting with activation of the plasma generator, as shown in FIG. 3.

FIG. 7 is a flow diagram of a method 700 comprising the salientoperations of the process to generate thrust via this illustrativeembodiment of the present invention. The figure outlines a number ofoperations and resultant effects that are interrelated as shown. Thehybrid engine in this illustrative embodiment of the present inventionexecutes the outlined operations and directly causes the indicatedeffects as a result of the recited operations. One or more components ofthe engine execute one or more of the recited operations, as discussedin more detail above.

In regard to method 700, it will be clear to those having ordinary skillin the art, after reading the present disclosure, how to make and usealternative embodiments of method 700 wherein the recited operations aredifferently sequenced, grouped, or sub-divided—all within the scope ofthe present invention. It will be further clear to those skilled in theart, after reading the present disclosure, how to make and usealternative embodiments of method 700 wherein some of the recitedoperations are optional, are omitted, or are executed by other elementsand/or systems associated with the engine; e.g., by elements that areexternal to and interconnected with the ion engine.

FIG. 8 shows two possible paths that plasma particles might follow whenescaping from plasma containment vessel 110 through outlets 150-1 and150-2. In particular, path 155-1 goes from point A to point B to pointC, while path 155-2 goes from point D to point E to point F.

FIG. 9a is a graph that shows the electrical potential along path 155-1from point A, through point B, to point C. In agreement with thepolarity shown in FIGS. 3-6, and 8 for electrodes 140-1, 140-2, 140-5,and 140-6, the electric potential first increases and passes through apositive peak, going from A to B, and then decreases and passes througha negative peak, going from B to C. This profile favors the flow ofpositive ions over negative electrons, as already remarked. The heightof the peaks is in proportion to the voltage provided by theconstant-voltage generator. The strength of the electric fieldsassociated with the electric potential shown in the figure is inproportion to the height of the peaks and, therefore, the effectivenessof the electric fields as a barrier to the flow of plasma ions can beadjusted as needed by adjusting the voltage provided by theconstant-voltage generator.

This is an important feature of the present invention, as the ability toadjust the height of the barrier independently of the kinetic energydelivered to the plasma particles, provides additional flexibility inadjusting the thrust generated by the engine.

FIG. 9b is a graph that shows the electrical potential along path 155-2from point D, through point E, to point F. In agreement with thepolarity shown in FIGS. 3-6, and 8 for electrodes 140-3, 140-4, 140-7,and 140-8, the electric potential first decreases and passes through anegative peak, going from D to E, and then increases and passes througha positive peak, going from E to F. This profile favors the flow ofnegative electrons over positive ions, as already remarked. Similarly tothe comment made for FIG. 9a , the height of the peaks is in proportionto the voltage provided by the constant-voltage and the effectiveness ofthe electric fields as a barrier to the flow of plasma electrons can beadjusted as needed by adjusting the voltage provided by theconstant-voltage generator.

FIGS. 3-6 show one laser 130 for generating laser beam 170. In someembodiment of the present invention, it is advantageous to use two ormore lasers. A second laser can be arranged to generate a second laserbeam that propagates in a direction parallel to the direction of laserbeam 170, but in the opposite direction. Such a second laser beam isadvantageous because second-order effects in the interaction betweenlaser beam 170 and plasma electrons might accelerate the electrons inthe direction of the laser beam. Such effects are likely to be small,compared to the predominant effect of the ponderomotive force which, asnoted above, accelerates the electrons in a direction perpendicular tothe direction of propagation of the laser beam. However, in embodimentswherein such second-order effects are undesirable, a second laserarranged as described above can reduce such effects.

A second laser is most effective if it generates a laser beam thatpropagates near laser beam 170. In particular, it is advantageous if thedistance between the two laser beams is, at most, five times thediameter of the larger of the two beams.

An important advantage of the present invention over the prior art isthat electrons in the plasma are accelerated preferentially over thepositive ions, and in a non-random fashion. This is in contrast to theprior-art technique of accelerating all plasma particles by heating theplasma. As noted above, the effect of the ponderomotive force is toaccelerate plasma electrons in a direction that is perpendicular to thedirection of propagation of the laser beam. Such directionality meansthat, by judiciously positioning the laser beam relative to outlets150-1 and 150-2, it is possible to enhance the proportion of electronsand positive ions that flow through the outlets. The shapes of the twooutlets and the position of the laser beam shown in FIGS. 4-6 have beendetermined to be advantageous through computer simulations. However, itwill be clear to those skilled in the art, after reading thisdisclosure, how to make and use embodiments of the present inventionwith other advantageous shapes for any number of outlets and with anynumber of laser beams advantageously positioned, relative to theoutlets, to take advantage of the directionality of the ponderomotiveforce.

It is to be understood that this disclosure teaches just one or moreexamples of one or more illustrative embodiments, and that manyvariations of the invention can easily be devised by those skilled inthe art after reading this disclosure, and that the scope of the presentinvention is defined by the claims accompanying this disclosure.

What is claimed is:
 1. An apparatus for imparting thrust to aspacecraft, the apparatus comprising: a plasma generator operable togenerate a plasma comprising generated ions; a containment vesseloperable to contain the plasma; a power supply operable to generatevoltage; a first outlet within the containment vessel; a second outletwithin the containment vessel; a first electrode positioned adjacent tothe first outlet, coupled to the power supply, and orientated such thatan electric field generated by the first electrode causes electrons tomove preferentially with respect to the first outlet such that some ofthe electrons escape from the containment vessel through the firstoutlet; and a second electrode positioned adjacent to the second outlet,coupled to the power supply, and oriented such that an electric fieldgenerated by the second electrode causes the generated ions to movepreferentially with respect to the second outlet such that some of thegenerated ions escape from the containment vessel through the secondoutlet.
 2. The apparatus of claim 1, wherein the electrons that escapethe containment vessel through the first outlet form a virtual cathodeoutside the containment vessel such that the virtual cathode generatesan electric field causing some of the generated ions to escape thecontainment vessel through the second outlet.
 3. The apparatus of claim1, wherein a constant charge is maintained with respect to the firstelectrode.
 4. The apparatus of of claim 1, wherein a constant charge ismaintained with respect to the second electrode.
 5. The apparatus ofclaim 1, wherein controlled operation of the plasma generator varies amagnitude of thrust imparted to the spacecraft.
 6. The apparatus ofclaim 1, wherein controlled operation of the first electrode and thesecond electrode varies a magnitude of thrust imparted to thespacecraft.
 7. The apparatus of claim 1, wherein controlled operation ofthe plasma generator varies a duration of thrust imparted to thespacecraft.
 8. The apparatus of claim 1, wherein controlled operation ofthe first electrode and the second electrode varies a duration of thrustimparted to the spacecraft.
 9. A method for imparting thrust to aspacecraft, the method comprising: generating a plasma, the plasmacomprising generated ions; containing the plasma within a confinedvolume of space; providing a first electrode associated with a firstoutlet, the first electrode oriented to generate an electric fieldcausing electrons within the confined volume of space to movepreferentially with respect to the first outlet such that some of theelectrons escape from the confined volume of space through the firstoutlet; providing a second electrode associated with a second outlet,the second electrode oriented to generate an electric field causing thegenerated ions within the confined volume of space to movepreferentially with respect to the second outlet such that some of thegenerated ions escape from the confined volume of space through thesecond outlet; coupling a power supply to the first electrode and to thesecond electrode; and controlling the first electrode and the secondelectrode to vary thrust imparted to the spacecraft.
 10. The method ofclaim 9, wherein the electrons that escape from the confined volume ofspace through the first outlet form a virtual cathode outside theconfined volume of space such that the virtual cathode generates anelectric field causing some of the generated ions to escape from theconfined volume of space through the second outlet.
 11. The method ofclaim 9, wherein a constant charge is maintained with respect to thefirst electrode.
 12. The method of claim 9, wherein a constant charge ismaintained with respect to the second electrode.
 13. The method of claim9, further comprising controlling the generation of plasma to varythrust imparted to the spacecraft.